1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to a driving integrated circuit (IC) unit for an LCD device.
2. Description of the Related Art
LCD devices having small size, lightweight, and low power consumption may solve problems related to cathode ray tube (CRT) devices. Accordingly, LCD devices have been a subject of recent research. The LCD device is a non-emissive display device including a liquid crystal layer sandwiched between array and color filter substrates. Of the different types of known LCD devices, active matrix LCD (AM-LCD) devices, which have thin film transistors and pixel electrodes arranged in a matrix pattern, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
The LCD device comprises an LCD module having a driving circuit unit and a backlight unit.
FIG. 1 illustrates a schematic cross-sectional view of a conventional LCD module.
In FIG. 1, a conventional LCD module 2 includes a liquid crystal display panel 10, a backlight unit 30, and a driver unit 50. The liquid crystal display panel 10 includes an upper substrate 12, also referred to as a color filter substrate, and a lower substrate 14, also referred to as an array substrate. A liquid crystal layer 15 is interposed between the upper and lower substrates 12 and 14 and a thin film transistor (TFT) “T”, to which a driving signal is applied from the driver unit 50, is formed on the lower substrate 14. The TFT “T” has a gate electrodes 16, source and drain electrodes 18 and 20, connected to a gate line, a data line (not shown) and a pixel electrode 22 applying a voltage to the liquid crystal layer 15, respectively. On the other hand, the driver unit 50 has a printed circuit board (PCB) 53 and a driving IC unit 54 including a driving IC 52.
Generally, the driving IC unit 54 is attached to the liquid crystal display panel 10 using an anisotropic conductive film. In conventional or low resolution type LCDs, the PCB having the driving IC can be easily attached to the liquid crystal display panel by a heat seal connector (HSC) because the driving IC does not have many leads. However, as the resolution of liquid crystal display panels increase, a driving IC has many leads and becomes harder to attach to the PCB. For example, a liquid crystal display panel having 600×800 pixels in super video graphics adapter (SVGA) requires 600×800×3 sub pixels, also known as dots, to display colors. All of the sub pixels need to be connected to the driving IC through a separate lead wire.
To solve the above-mentioned problems, a tape automated bonding (TAB) has been suggested. A TAB type driving IC is easily attached to a high resolution liquid crystal display by loading the TAB type driving IC onto a tape carrier. As shown in FIG. 1, the TAB type driving IC is loaded onto a tape carrier package (TCP) and is mainly used within the driving IC unit 54. Recently, a chip on flexible printed circuit (COF) type driving IC has been researched. Since the COF type driving IC is loaded on a more flexible film than in the TCP type driving IC, the thickness of the COF type driving IC unit is reduced so that a fine pad pitch may be formed.
FIG. 2 illustrates a schematic plan view showing a relationship between a driving IC and an LCD panel of a conventional LCD module.
In FIG. 2, gate and source electrodes 16 and 18 of the LCD module 2 are connected to gate and data lines 17 and 19, respectively. In a pixel region defmed by the gate and data lines 17 and 19, red (R), green (G), and blue (B) sub pixels are sequentially disposed to compose one pixel “P”.
The driving IC 52 functions as an interface between the LCD panel 10 and the driving unit. The driving IC 52 includes a gate driving IC 52a and a source driving IC 52b. The gate driving IC 52a is disposed at each of the right and left sides of the LCD panel 10 and turns a TFT “T” on or off by sequentially applying a driving voltage to the gate line 17. The source driving IC 52b is disposed at each of the top and bottom of the LCD panel 10 and applies a signal voltage to the data line 19 and transfer a data voltage to a liquid crystal layer through the TFT turned on.
The LCD module 2 is driven by alternating current (AC) to prevent direct current (DC) from weakening the liquid crystal layer the LCD. The AC driving method also prevents a degradation of display quality (e.g., flicker) due to a pixel voltage variation according to a field and a residual image that occurs when a still image is displayed for a long time. For the AC driving method, a line inversion driving method is mainly used. In the line inversion driving method, positive (+) signals are applied to the data lines when one gate line is turned on and negative (−) signals are applied to the data lines when the next gate line is turned on.
At the ends of the gate and data lines 17 and 19, gate and data pads (not shown) are formed, respectively. A plurality of output pads corresponding to the gate and data pads are formed in the driving IC 52.
FIG. 3 illustrates a schematic plan view of a conventional driving IC unit of a COF type for a LCD device.
In FIG. 3, the driving IC unit 54 includes a base film 58 and a plurality of input lines 68 and output lines 70 are formed on the base film 58 along a first direction. At one end of each input line 68 and each output line 70 near a boundary of the base film 58, input pads 64 and output pads 66 are disposed, respectively. At the other end of each input line 68 and each output line 70, in a middle of the base film 58, input terminals 60 and output terminals 62 are disposed, respectively. The input pad 64 is connected to a PCB (not shown) and an input signal is transferred from the PCB to the input pad 64. The output pad 66 is connected to gate or data pads of an LCD panel as illustrated in FIG. 2. Furthermore, a driving IC 56 connected to the input and output terminals 60 and 62 is formed on the base film 58.
Generally, since the resolution of an LCD module is determined by a number of gate and data lines, a compact LCD module of high resolution can be realized by forming a driving IC having a fine pad pitch. The minimum pad pitches of TCP type and COF type driving ICs are within a range of about 70 to about 75 micrometers and about 50 micrometers, respectively. Since the COF type driving IC is thinner than that of the TCP type, the distance between pads may be reduced due to a taper angle present in the COF type driving IC.
FIG. 4 illustrates a schematic cross-sectional view taken along a line IV—IV of FIG. 3. A base film and a driving IC are not shown for convenience of explanation. Since an output terminal has a size corresponding to an output pad, the pitch between output terminals may be referred as a pad pitch.
In FIG. 4, the pad pitch “pp” may be expressed as follows.1pp(50 μm)=2a(30 μm)+d(20 μm)
where “a” is a half width of the output terminal 62 and “d” is a distance between output terminals 62.
The degree of pad pitch may be measured by calculating the number of the output lines per inch (2.54 cm=25400 μm) capable of being designed using one pad pitch “pp”. Since the number of output lines corresponding to one pad pitch “pp” is one, the number of output lines per inch is 508. (25400 μm/50 μm=508 EA/inch) Since one pixel is composed of three sub pixels, the number of output terminals per inch corresponds to a maximum of 170 pixels/inch. (508/3=170)
It is difficult to form LCD devices having a display of 200 pixels per inch (PPI) while employing COF type driving IC units according to the single bank method, where source driving ICs are disposed at only one side of the LCD panel. Even though the pixel pitch of the 200 PPI LCD device is calculated as 42 μm, the pad pitch, being narrower than the pixel pitch (e.g., less than 40 μm), is conventionally difficult to fabricate.
Since clock frequency increases as operating speed and resolution increases, driving ICs having corresponding clock frequencies for ultra high resolution LCD devices (e.g., over quadruple extended graphics adapter (QXGA) having 2048×1536 pixels) are not easily fabricated according to the single bank method. To improve the problems associated with the single bank method, a double bank method has been suggested. In the double bank method, data driving IC units are disposed at both sides of the LCD panel.
FIG. 5 illustrates a schematic plan view showing an LCD module of a conventional double bank method.
In FIG. 5, the LCD module of the conventional double bank method processes signals at driving IC units 72 of both sides so that high speed driving can be achieved at high resolution. However, as the area of the driving IC units 72 increases and processes of the driving IC and module become complex, the LCD device driven according to the double bank method is not easily made compactly is associated with high fabrication costs.